Strain-driven electronic band structure modulation of si nanowires

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.     

Diameter and polarization-dependent Raman scattering intensities of semiconductor nanowires

Diameter-dependent Raman scattering in single tapered silicon nanowires is measured and quantitatively reproduced by modeling with finite-difference time-domain simulations. Single crystal tapered silicon nanowires were produced by homoepitaxial radial growth concurrent with vapor-liquid-solid axial growth. Multiple electromagnetic resonances along the nanowire induce broad band light absorption and scattering. Observed Raman scattering intensities for multiple polarization configurations are reproduced by a model that accounts for the internal electromagnetic mode structure of both the exciting and scattered light. Consequences for the application of Stokes to anti-Stokes intensity ratio for the estimation of lattice temperature are discussed.     

Fabrication of reliable semiconductor nanowires by controlling crystalline structure

One-dimensional SnO(2) nanomaterials with wide bandgap characteristics are attractive for flexible and/or transparent displays and high-performance nano-electronics. In this study, the crystallinity of SnO(2) nanowires was regulated by controlling their growth temperatures. Moreover, the correlation of the crystallinity of nanowires with optical and electrical characteristics was analyzed. When SnO(2) nanowires were grown at temperatures below 900?°C, they showed various growth directions and abnormal discontinuity in their crystal structures. On the other hand, most nanowires grown at 950?°C exhibited a regular growth trend in the direction of [100]. In addition, the low temperature photoluminescence measurement revealed that the higher growth temperatures of nanowires gradually decreased the 500 nm peak rather than the 620 nm peak. The former peak is derived from the surface defect related to the shallow energy level and affects nanowire surface states. Owing to crystallinity and defects, the threshold voltage range (maximum-minimum) of SnO(2) nanowire transistors was 1.5 V at 850?°C, 1.1 V at 900?°C, and 0.5 V at 950?°C, with dispersion characteristics dramatically decreased. This study successfully demonstrated the effects of nanowire crystallinity on optical and electrical characteristics. It also suggested that the optical and electrical characteristics of nanowire transistors could be regulated by controlling their growth temperatures in the course of producing SnO(2) nanowires.     

Unique electronic band structures of hydrogen-terminated [Formula: see text] silicon nanowires

Band structure mutation from an indirect to a direct gap is a well-known character of small hydrogen-terminated [Formula: see text] and [Formula: see text] silicon nanowires (SiNWs), and suggests the possible emission of silicon. In contrast, we show that hydrogen-terminated [Formula: see text] SiNWs consistently present indirect band gaps even at an extremely small size, according to our calculations using density functional theory. Interestingly, the band gap of [Formula: see text] SiNWs shows a quasi-direct feature as the wire size increases, suggesting the possibility of using medium SiNWs in optoelectronic devices. This result also indicates that the electronic structures of SiNWs are strongly orientation dependent.     

Photomodulated reflectance and transmittance: optical characterisation of novel semiconductor materials and device structures

Photomodulation spectroscopy, in reflection and transmission modes, is presented here as a powerful non-destructive optical technique for the investigation of fundamental physical properties of new semiconductor materials and complex micro- and nano-structures. The abilities of photoreflectance and phototransmittance in application to many kinds of semiconductor structures are demonstrated. The following aspects are discussed: (1) separation of the optical response and built-in electric field determination in different depths of the sample by a selection of the pump beam wavelength; (2) electric field in δ-doped structures by an application of a fast Fourier transformation; (3) electron concentration dependence of the band gap related transitions in wurtzite GaN epitaxial layers; (4) comparison of different spectroscopic techniques used for investigations of InGaSb/GaSb quantum wells within 1.5-2 μm spectral region; (5) quantum well intermixing effects in InGaAsP/InP 1.55-μm laser structures; (6) photomodulation spectroscopy of self-assembled quantum dots.     

High-T c superconductivity and electronic band structure

We review the main results of the van Hove scenario applied to superconducting cuprates. It is based on the assumption that in these materials, the Fermi level lies near a singularity in the density of states (DOS). This hypothesis has recently been confirmed experimentally. We show that this model explains many properties of the high-T _c superconductors. We show that an anaogous model with a peak in the DOS may also be applied to the superconducting doped fullerenes. A general feature of the model is a very short coherence length.     

How strain controls electronic linewidth in single beta-phase polyfluorene nanowires

Low-temperature single-molecule fluorescence spectroscopy reveals pure, virtually defect-free chains of the one-dimensional crystalline beta-phase of polyfluorene. The likelihood of beta-phase formation is shown to correlate directly with the initial shape of the polymer chain, with extended chains preferentially forming this planarized phase. Planarized chains, characterized by a distinct spectroscopic signature can, however, exhibit substantial bending within the plane. This bending results in a strong increase in the elementary transition linewidth of the conjugated segment. The transition linewidth provides a lower limit to the electronic dephasing time of the excited state of >3 ps at 5 K. Remarkably, bending does not appear to disrupt the pi-electron conjugation so that the emission from a single bent beta-phase chromophore is not necessarily linearly polarized as is generally assumed.     

Coherent scattering theory for the electronic band structure and quantum-state lifetimes in a dilute substitutional Mg alloy

The virtual crystal approximation is used to formulate the Schrödinger equation for the electronic band structure of a dilute substitutional alloy and to analyze the effect of coherent scattering on the lifetime _k of an electron quantum state. Both coherent and incoherent scattering theory yield essentially the same functional and numerical relationship connecting _k ,C, and |V _O|, whereC is the impurity concentration and |V _O| is the typical Fourier transform of the impurity perturbation potential. Electron quantum interference data for Mg-Zn and Mg-Cd alloys are interpreted using this formulation. A consistent picture of these alloys results which contains the following features. (1) The impurity atoms in both types of alloys are partially ordered in the Mg lattice. (2) The perturbation potentials for both Zn and Cd are somewhat larger than predicted by simple pseudopotential theories, with the perturbation for Zn being about 6.5 times larger than for Cd. (3) A standing charge density wave commensurate with the lattice and having aC-dependent amplitude is present in these alloys.Work supported by the National Science Foundation.Fannie and John Hertz Foundation Fellow.     

镍间隙掺杂硅纳米线电子结构的研究

采用基于密度泛函理论的第一性原理的方法,对[100]方向镍间隙掺杂硅纳米线结构的稳定性和电子性质进行了计算。计算结果表明Ni原子更喜欢占据硅纳米线内部六角形间隙位置;掺杂体系费米能级附近的电子态密度来源于Ni3d态电子的贡献;同时发现不同构型的Ni掺杂硅纳米线,其带隙不同,且与未掺杂硅纳米线相比,带隙普遍减小。     

Raman scattering and growth disorders in single as-grown TiO2 nanowires

An oxidation procedure has been developed to grow single-crystalline TiO(2) nanowires of the pure rutile phase, allowing subsequent characterizations of SEM, XRD, Raman, and TEM without any post-growth preparations. TEM observations support that the 1D anisotropic growth is dominated by oriented attachment processes, leading to typical growth-induced defects in the nanowires. Spatial variations of the rutile E(g) and A(1g) Raman modes were unambiguously revealed on single nanowires while scanned along the growth direction parallel to the rutile [110]. Symmetry-sensitive deviations were identified by comparing the Raman data with the spatial correlation model calculations based on realistic dispersion relations of the rutile, reflecting morphology-correlated defect distributions along single nanowires. This work provides an efficient, non-destructive in situ characterization approach for guiding growth design in future nanotechnology.     

Electronic properties of semiconductor nanowires

This paper provides a review of the state-of-the-art electronic-structure calculations of semiconductor nanowires. Results obtained using empirical k.p, empirical tight-binding, semi-empirical pseudopotential, and with ab initio methods are compared. For conciseness, we will restrict our detailed discussions to free-standing plain and modulated nanowires. Connections to relevant experimental data, particularly band gaps and polarization anisotropy, will be made since these results depend crucially on the electronic properties. For completeness, a brief review on the synthesis of nanowires is included.     

Surface-enhanced Raman scattering imaging of a single molecule on urchin-like silver nanowires

Urchin-like silver nanowires are prepared by reacting AgNO(3)(aq) with copper metal in the presence of cetyltrimethylammonium chloride and HNO(3)(aq) on a screen-printed carbon electrode at room temperature. The diameters of the nanowires are about 100 nm, and their lengths are up to 10 μm. Using Raman spectroscopy, the detection limit of Rhodamine 6G (R6G) on the urchin-like silver nanowire substrate can be as low as 10(-16) M, while the analytical enhancement factor is about 10(13). Raman mapping images confirm that a single R6G molecule on the substrate can be detected.     

Direct imaging of graphene edges: atomic structure and electronic scattering

We report an atomically resolved scanning tunneling microscopy investigation of the edges of graphene grains synthesized on Cu foils by chemical vapor deposition. Most of the edges are macroscopically parallel to the zigzag directions of graphene lattice. These edges have microscopic roughness that is found to also follow zigzag directions at atomic scale, displaying many ～120° turns. A prominent standing wave pattern with periodicity ～3a/4 (a being the graphene lattice constant) is observed near a rare-occurring armchair-oriented edge. Observed features of this wave pattern are consistent with the electronic intervalley backscattering predicted to occur at armchair edges but not at zigzag edges.     

Confinement-guided shaping of semiconductor nanowires and nanoribbons: "writing with nanowires"

To fully exploit their full potential, new semiconductor nanowire building blocks with ab initio controlled shapes are desired. However, and despite the great synthetic advances achieved, the ability to control nanowire's geometry has been significantly limited. Here, we demonstrate a simple confinement-guided nanowire growth method that enables to predesign not only the chemical and physical attributes of the synthesized nanowires but also allows a perfect and unlimited control over their geometry. Our method allows the synthesis of semiconductor nanowires in a wide variety of two-dimensional shapes such as any kinked (different turning angles), sinusoidal, linear, and spiral shapes, so that practically any desired geometry can be defined. The shape-controlled nanowires can be grown on almost any substrate such as silicon wafer, quartz and glass slides, and even on plastic substrates (e.g., Kapton HN).     

Chemical probing of single cancer cells with gold nanoaggregates by surface-enhanced Raman scattering

By using near-infrared surface-enhanced Raman scattering (SERS) with 60 nm gold nanoparticles (Au-NPs) to probe the chemical composition inside single human osteosarcoma cells we have shown that the SERS intensity may increase by a factor of 3-6 times in different parts of the cells depending on the density of gold nanoaggregates within the probed volume after the cell is dehydrated. The cellular points of low-density gold nanoaggregates exhibit more significant increase of SERS signal levels, the cellular macrochemicals such as nucleic acids show conformational changes, and new components can be probed after the cell is completely dried. A comparative study between viable and apoptotic cells indicates that most of the Au-NPs that enter the living cell reside in the cytoplasm and around the nucleus, whereas glyoxal-induced apoptotic cells show relatively uniform distribution of Au-NPs and, interestingly, the presence of DNA fragments is detected throughout the cell, including the cell surface.     

Optical coupling of deep-subwavelength semiconductor nanowires

Systems of coupled resonators manifest a myriad of exciting fundamental physical phenomena. Analogous to the synthesis of molecules from single atoms, the construction of photonic molecules from stand-alone optical resonators represents a powerful strategy to realize novel functionalities. The coupling of high quality factor (Q) dielectric and semiconductor microresonators is by now well-understood and chipscale applications are abound. The coupling behavior of low-Q nanometallic structures has also been exploited to realize high-performance plasmonic devices and metamaterials. Although dense arrays of semiconductor nanoparticles and nanowires (NWs) find increasing use in optoelectronic devices, their photonic coupling has remained largely unexplored. These high refractive index nano-objects can serve as low-Q optical antennas that can effectively receive and broadcast light. We demonstrate that the broad band antenna response of a pair of NWs can be tuned significantly by engineering their optical coupling and develop an intuitive coupled-mode theory to explain our observations.     

Surface exciton-plasmon polariton enhanced light emission via integration of single semiconductor nanowires with metal nanostructures

The light emission enhancement behavior from single ZnO nanowires integrated with metallic contacts is investigated by micro-photoluminescence measurements. Apart from surface plasmon polaritons at the air/metal interface, the emission of a single ZnO nanowire can be coupled into guided modes of surface excitonplasmon polaritons (SEPPs). The out-coupling avenues of SEPP guided modes are modeled in the presence of nanostructures, such as corrugation and gratings, on the metal surface. The guided modes of SEPPs in metalcontacted ZnO nanowires are calculated using the effective index method. The enhanced light emission from single semiconductor nanowires shows promise for use in highly efficient nano-emitters and nano-lasers, as well as macroscopic solid state light sources with very high efficiency. This article is published with open access at Springerlink.com     

Three-dimensional nanoscale composition mapping of semiconductor nanowires

We demonstrate the three-dimensional composition mapping of a semiconductor nanowire with single-atom sensitivity and subnanometer spatial resolution using atom probe tomography. A new class of atom probe, the local electrode atom probe (LEAP) microscope, was used to map the position of single Au atoms in an InAs nanowire and to image the interface between a Au catalyst and InAs in three dimensions with 0.3-nm resolution. These results establish atom probe tomography as a uniquely powerful tool for analyzing the chemical composition of semiconductor nanostructures.